2-Alkylpyrimidines, particularly 2-tert-butylpyrimidine, are advantageously employed as intermediates for the preparation of certain organophosphorus insecticides as described in U.S. Pat. No. 4,127,652.
The preparation of 2-alkylpyrimidines is taught in U.S. Pat. No. 3,050,523. That process requires the reaction of an alkylene 1,3-diamine with an organic carboxylic acid, ester or amide over a supported noble metal catalyst having dehydration and dehydrogenation activity. More recently, the individual steps (a-c) of the conversion of 1,3-diaminopropane and an alkanecarboxylic acid to a 2-alkylpyrimidine have been investigated; see, for example, U.S. Pat. Nos. 4,376,201; 4,493,929; 4,880,929; and 4,999,427; and European Patent Application Publication Nos. 117,882; 192,297; and 192,299.
a) Amidation ##STR1## b) Cyclization/Dehydration ##STR2## c) Dehydrogenation ##STR3##
U.S. Pat. No. 4,376,201 describes the vapor phase reaction wherein a 3-aminopropyl carboxylic acid amide is cyclized and dehydrogenated over a supported platinum or palladium catalyst (steps b+c).
U.S. Pat. No. 4,493,929 describes the separation of steps b+c and the improved vapor phase reaction wherein 2-alkylpyrimidines are obtained in high yield and purity by the dehydrogenation of a 2-alkyltetrahydropyrimidine under conditions which do not generate water and in which no water is added (step c).
European Patent Application Publication No. 117,882 describes an overall process for the preparation of 2-tert-butylpyrimidine which comprises the following individual steps: a) the preparation of 3-aminopropyl pivalamide by reaction of pivalic acid and an excess of 1,3-diaminopropane; b) removal of unreacted 1,3-diaminopropane by distillation and dehydration of the 3-aminopropyl pivalamide to 2-tert-butyl-1,4,5,6-tetrahydropyrimidine in the liquid phase, preferably in the presence of a solvent capable of azeotroping water; and c) the dehydrogenation of 2-tert-butyltetrahydropyrimidine to 2-tert-butylpyrimidine, under conditions in which water is neither generated nor added, over a supported noble metal catalyst.
European Patent Application Publication No. 192,297 describes the vapor phase dehydrogenation of 2-propyl- or 2-butyl-1,4,5,6-tetrahydropyrimidine over a palladium-containing catalyst in which the catalyst lifetime is prolonged by operating the presence of carbon monoxide and hydrogen (step c).
European Patent Application Publication No. 192,299 describes the multistep preparation of 2-methyl and 2-ethylpyrimidine by: the reaction of an acetic or propionic acid derivative with 1,3-diaminopropane in the liquid phase to form a 1-amino-3-amidopropane (step a); optional cyclization to the 2-methyl- and 2-ethyltetrahydropyrimidine (step b); and optional cyclization and gas phase dehydrogenation to 2-methyl- and 2-ethylpyrimidine with a palladium-containing catalyst in the presence of carbon monoxide and hydrogen (step c or steps b+c).
U.S. Pat. No. 4,880,929 describes an improved stepwise, continuous process for the preparation of a 2-alkylpyrimidine from the appropriate alkanecarboxylic acid and a 1.5 to 2.5 fold excess of 1,3-diaminopropane. In a very similar process, U.S. Pat. No. 4,999,427 describes preferred ratios of diamine to alkanecarboxylic acid of 1.1:1 to 6:1. In addition, this latter patent suggests that unreacted alkanecarboxylic acid has a deleterious effect on the lifetime of the palladium-containing dehydrogenation catalyst.
With time, the dehydrogenation catalysts typically lose their efficiency and must be replaced or regenerated Because supported palladium catalysts are prone to sintering, they are often regenerated off-site and the palladium must be reprocessed. Off-site regeneration means the undesirable shutdown of the process followed by cooling and down-loading of the catalyst. Therefore, it is highly desirable to increase the effective lifetime of the catalyst in order to extend the time between catalyst regenerations. It is also desirable to have an on-line or on-site regeneration process to minimize disruption of the operation.